The invention relates to a DC to DC converter. More particularly, the invention relates to a DC/DC converter usable in bidirectional energy transfers to interconnect two different DC voltage sources and networks. More particularly, the invention relates to a DCxe2x80x94DC converter that can be used in automobiles having a 42 volt system or a dual 42v/14v system. The invention allows interfacing between 14v and 42v systems. For example, the invention can allow 14v systems and 42v systems to supply each other, as needed. The invention would also be used to allow a 14v system to charge or jump start a 42 volt system, or vice versa.
It is known to have two DC voltage networks coexistent and cooperating within a single system. For example, it is expected that automobiles may be provided with a 14 volt network for driving a first set of loads and a 42V network for powering further loads. It is sometimes necessary to provide added energy to the 12 volt or 42 volt system. In that case the system must be separately charged; or must be sufficiently large that it can supply any excess demand.
It would be desirable to provide a single DC/DC converter which allows two different DC voltage networks to be interconnected in an efficient and simple manner and which allows bi-directional energy transfer between the networks and associated loads.
It is also desirable to provide a converter which allows a 14v system to charge or jump start a 42v system and vice versa.
Further, it is desirable to provide a converter which minimizes load dumps, i.e., minimizes the storage and release of voltage spikes from for example, the vehicle alternator or other sources.
In accordance with the invention, a DC/DC bidirectional converter is provided, including a transformer and two pairs of alternatively operating semiconductor switches for transferring electrical energy between two-sources of energy of different voltages, e.g. 12 volts and 42 volts, and their loads. The novel circuit allows either of two different power sources to drive the other source and load.
More specifically, the converter may comprise four switches, preferably vertical conduction MOSFETs, in a modified H bridge type configuration. A multi-turn auto transformer is connected to the bridge. The center of the transformer winding is connected to one of the voltage sources of the system. Two taps between the center and ends of the windings are connected to respective low side MOSFETs while the high side MOSFETs are connected between respective winding ends and the second voltage source. Diagonally disposed switches are then simultaneously and alternately turned on and off, with an approximate 50% duty cycle, with energy being transferred between the two networks by the transformer action of the auto transformer.
The converter of the present invention has no voltage control of its own but operates as a true dc-to-dc bi-directional converter of whatever voltage the automotive alternator dictates. The alternator""s output can be located on the 14-volt bus or on the 42-volt bus. Preferably, the alternator will supply the 42v bus.
The novel converter has a current limiter to protect itself against excessive currents. The current measurement needs only to be on one of the 14v or 42V connections because the current is common to both voltage rails but is of different magnitudes.
The converter of the present invention operates with a square wave and very little dead time. The transformer is wound with a 3:1 turns ratio, connected as an autotransformer connection to minimize components as well as reduce peak currents by ⅓. The 3:1 ratio operates similar to a standard ac transformer with isolated windings and acts as a constant power converter (e.g., if 5 amps are applied to the 42 volt bus and there is no load there, then the load must be on the 14 volt bus, but the current is now (3xc3x975) or 15 amps). The time base can be derived from various dual output pwm controllers such as the SG1525, which also provides a proportional volt second control for each side of the transformer so as to prevent core saturation. The pairs of diagonal MOSFET switches of the bridge switch simultaneously and alternatingly with the other pair. The MOSFETs operate as a switch and as a rectifier at the same time (synchronous rectification). There are no pulsating currents on the input or on the output when operating within the current rating range. During the start-up and if energy is needed in excess of the capability of the converter, it has a current limiting PWM mode of operation. PWM is also used during the start up (soft start is built into the SG1525). An inductor may be used during the PWM period. A resistor may be used to measure current by way of a comparator such as the LM339 and translates to the PWM controller via another switching device. Alternatively, a current transformer can be used. The control circuit is referenced to a virtual earth ground, which is developed by diodes. A capacitor and diodes also form a clamp for the only winding (i.e., the then unconnected winding) needing an energy clamp. The virtual earth ground is approximately 15 volts below the traditional automotive earth ground. This allows the use of four of the isolated gate drivers such as the IR2110. Each of the IR2110xe2x80x2s is used to drive one of the gates of the MOSFETs. The dead time needed to insure safe operation is a feature of the SG1525.
When current is flowing into or out of the 42-volt bus, its magnitude is equal in each and alternating in the two high side MOSFETs. When current is going into or out of the 14-volt bus, its magnitude is shared ⅓ in a high side MOSFET, and ⅔ in a low side MOSFET. This is from the transformer action of the autotransformer connection.
To provide a reliable switching operation, in accordance with another aspect of the invention, the DC/DC converter has a control unit. This unit includes a pulse width modulator (PWM) generating a pair of control signals applied to the gates of semiconductor switches in a desired manner, and a set of amplifiers and drivers for converting the generated control signals to required levels.
A further aspect of the invention relates to a current limiting circuit operating to monitor the currents flowing across a current sensor and to limit load currents at a certain safe level to prevent the electronic components and/or the load from structural damage. The same circuit provides a soft start of the converter.
An important feature of the invention is that it minimizes load dump because it does not include, at least in the power conversion circuitry, any energy storage components such as large inductors or capacitors. This helps to reduce the occurrence of load dumps into a vehicle""s electronic circuitry due to stored spikes from, e.g., the vehicle alternator or other sources (e.g. jump starts).
Another feature is that the converter provides for current/temperature compensation at one of the networks, e.g., the 42v network, which is automatically replicated at the other network, e.g., the 14v network. Further, in the converter of the invention, there is no interruption of current during operation, minimizing the occurrence of harmful transients.
Other features and advantages of the present invention will become apparent from the following description of the invention which refers to the accompanying drawings.